1. Field of the Invention
The present invention relates to a process for producing phthalic anhydride using a fixed bed reactor. Particularly, the invention relates to a process for producing phthalic anhydride in a high yield by gas-phase catalytic oxidation of ortho-xylene and/or naphthalene with molecular oxygen or an molecular-oxygen-containing gas using a fixed bed reactor.
2. Description of the Related Art
Phthalic anhydride is produced by a process of gas-phase catalytic oxidation of ortho-xylene and/or naphthalene with molecular oxygen or a molecular-oxygen-containing gas (these are hereinafter briefly referred to as xe2x80x9coxygen-containing gasxe2x80x9d). In this process, the use of catalysts each containing vanadium oxide and titanium oxide as active ingredients supported on an inert carrier is widely known, and is described in, for example, Japanese Examined Patent Application Publications No. 47-15323, No. 49-41036, and No. 52-4538, and Japanese Unexamined Patent Application Publications No. 47-5661, No. 49-89694, and No. 57-105241. These catalysts each have a feature and some of them have been used in commercial production.
However, the performances of such catalysts are susceptible to improvement. In consideration of the scale of production system, even a 1% increase in yield of phthalic anhydride by an improvement of a catalyst can give large economic effects. An improvement in selectivity can facilitate heat treatment and distillation procedure to yield a final product and is thereby expected to produce high quality products at low costs.
In addition, it is important to ensure a stable production through an increased productivity and a durable catalytic activity. A possible solution to increase the productivity is to conduct an oxidation reaction under high loading conditions. For example, the concentration of a raw material gas (ortho-xylene and/or naphthalene) relative to an oxygen-containing gas is to be increased. However, a reaction of ortho-xylene and/or naphthalene to yield phthalic anhydride is a highly exothermic reaction, and under such high loading conditions, the temperature of a hot spot vigorously increases and an excessive oxidation reaction occurs to markedly deteriorate the catalyst. As a result, the yield of phthalic anhydride is decreased. If the temperature of the hot spot excessively increases, heat of reaction cannot be sufficiently removed, which will induce a runaway reaction.
Catalysts having a satisfactory heat resistance and stable toward the use under such high loading conditions have been proposed, for example in Japanese Examined Patent Application Publication No. 59-1378. However, even when such catalysts are used, decrease in the yield of phthalic anhydride, deterioration of catalyst and the danger of a runaway reaction cannot be avoided unless excessive temperature rise in a hot spot is controlled.
Under these circumstances, to performed operations under high loading conditions, an attempt has been made to inhibit the temperature rise in the hot spot by configuring the catalytic layer as two or more individual catalytic layers having different catalytic activities. However, if conversion of a raw material in individual catalytic layers is out of valance, the temperature in a hot spot still excessively vigorously rises to thereby invite an excessive oxidation reaction. As a result, this configuration cannot also solve the problems of decrease in yield of phthalic anhydride and deterioration of the catalyst. In extreme cases, a runaway reaction in the hot spot will occur.
Accordingly, an object of the invention is to provide a process for producing phthalic anhydride, which is capable of providing phthalic anhydride in a high yield, is capable of minimizing deterioration of catalysts with time, and is capable of continuously stably producing phthalic anhydride, even when a high concentration raw material gas is fed.
Specifically, the invention provide a process for producing phthalic anhydride through gas-phase catalytic oxidation of ortho-xylene and/or naphthalene with an oxygen-containing gas using one or more fixed bed reactors. In the process, the gas-phase catalytic oxidation process is performed in three or more individual catalytic layers, and the conversion rates of ortho-xylene and/or naphthalene in the individual catalytic layer are defined within specific ranges.